Molecular sieves, including zeolites, have very small pores, the size of which is uniform within a variation of 0.1 Å and the shape of which varies depending on the framework structure of the molecular sieves, and thus these molecular sieves show unique shape-selective properties which are not observed in amorphous oxides. Accordingly, these nanoporous materials have been used as ion exchangers, separating agents, catalysts or catalyst supports, in the fine chemical industry, the petrochemical industry, and the like [Kirk Othmer Encyclo. Chem. Technol., 1996, Vol. 16, pp. 888]. The successful use of such nanoporous materials can be because new zeolites whose framework structures or compositions differ from those of existing materials have been continuously developed for several years, thus making it possible to significantly improve many commercially important chemical processes. Gallosilicate zeolites comprising gallium and silicon as framework elements have physicochemical and catalytic properties different from those of aluminosilicate zeolite [Oil Gas J., 1985, Vol. 83, pp. 1288], and thus studies on the synthesis thereof have been actively conducted, and more than 20 types of gallosilicate zeolites having different framework structures have been reported to date [Chem. Rev. 2000, Vol. 100, pp. 2303].
Accordingly, the present inventor has conducted many studies to synthesize a molecular sieve having a framework structure and composition different from those of known zeolites and molecular sieves. As a result, the present inventor has discovered one type of gallosilicate zeolite having a novel composition and has found that this type of zeolite has a significant effect on gas adsorption.
High-purity hydrogen can be used in almost all fields, including industrial basic materials and current energy systems such as general fuels, semiconductors and fuel cells, and thus has received attention as a next-generation energy source that can substitute for existing energy systems based on electricity and gases. In order to achieve a hydrogen energy system different from existing energy systems, a technology for producing large amounts of hydrogen should be provided, because the hydrogen energy system should be used in commercial and domestic applications in addition to existing industrial applications. In long-term economic terms, the production cost of hydrogen should be lower than the production cost of fossil fuels. Known methods for producing large amounts of hydrogen include: (1) a method of producing hydrogen by electrolysis of water, and (2) a method of producing hydrogen by steam reforming. The electrolysis method (1) will not be cost-effective if high power is used, because the production costs of the products (hydrogen and oxygen) greatly depends on the amount of power. For this reason, studies on systems, which produce inexpensive power by renewable energy such as wind force or solar energy and produce hydrogen by electrolysis using the produced power, have recently been conducted, but such systems are difficult to use in practice due to many technical problems. Thus, under current circumstances, the steam reforming method (2) is the most suitable method for producing large amounts of hydrogen. The steam reforming method is a method of obtaining hydrogen gas from natural gas and heavy naphtha which is a byproduct of an oil refining process. In the steam reforming process, such hydrocarbons are converted into synthesis gases, such as CO, CO2 and H2, by allowing them to react with steam at a high temperature of about 800° C. using nickel oxide as a catalyst. Herein, the obtained synthesis gases are treated by a gas separation process depending on the intended use thereof. In order to obtain high-purity hydrogen, CO among the synthesis gases obtained by the reforming process is separated into CO2 and H2 by a shift conversion reaction, after which CO2 is separated and H2 is separated and purified from impurities (such as CO, H2O, CH4, etc.) remaining in the reaction system.
Typical processes for the separation and purification of hydrogen include a pressure swing adsorption (PSA) process and a membrane separation process. In the pressure swing adsorption process, raw material gas is passed through an adsorption column packed with an adsorbent under high pressure, while components with high selectivity are adsorbed onto the adsorbent, and desired components with low selectivity are discharged from the adsorption column. However, in this process, an equilibrium reaction occurs at a high temperature of about 800° C., a large-scale complicated system is used, the number of treatment processes and apparatuses is increased, a high installation cost is required, and the maintenance and repair of apparatuses are difficult. In addition, high-purity hydrogen is not obtained, thus making it difficult to provide a sufficient amount of high-purity hydrogen. Meanwhile, the membrane separation processes which are currently used in practice mostly perform the separation and purification of hydrogen through a Pd—Ag alloy membrane, a hydrogen-permeable alloy membrane. However, Pd is expensive and is not abundant in nature, and thus the development of a material to substitute for Pd is requested.
Meanwhile, processes of separating gases from each other using zeolites have been attempted. Zeolite is crystalline, and thus has a uniform structure and a uniform pore size and shape. The pore opening size of zeolite is primarily determined according to the framework structure, also varies depending on the kind and degree of exchange of cations. Among the selective adsorption properties of zeolite, the most important is that a material larger than the pore opening cannot enter the pore so that it is not adsorbed onto zeolite. Different gases have different molecular sizes, and different zeolites also have different pore opening sizes. Thus, when zeolite is suitably selected, it can separate a gas mixture into components according to molecular size. Also, due to the energy interaction between cations and polar or polarizable adsorbents, these adsorbents are adsorbed with a mixture of chemical species having low polarity or polarizability. This molecular sieve effect is an inherent property which appears because zeolite is a crystalline material and has a pore size similar to the molecular size thereof, and this molecular sieve effect is effectively used in the separation and purification field. However, a zeolite that selectively adsorbs hydrogen gas has not yet been found.